It has been known for a long time that glazing units and mirrors may be electrically heated by providing them an electrically-conducting coating and/or electrically-conducting tracks and by heating the coating and/or the conducting tracks by applying an electrical voltage and allowing a current to flow. The heat produced allows visual impairments caused by condensation of water vapour and/or frosting up or snow to be very quickly eliminated from such glazing units. On the glazing units of a vehicle, frozen windshield wipers can be freed up.
In the field of layered heaters for glazing units, an arrangement (U.S. Pat. No. 2,878,357) is known in which an electrically-conducting transparent coating on a glazing unit with a trapezoidal shape is divided into several strips disposed adjacent to one another which are electrically connected in series with one another by means of collector conductors respectively disposed alternately on the top and bottom edge of the strips. Thus, a voltage divider is produced; the same current flows in all the strips with different voltage drops. The result is that the total current that flows (equally in the short strips having relatively low resistances) is limited to the value that the strip with the highest resistance (where the voltage drop is the greatest) still allows to flow.
The Patent Application DE 36 44 297 A1 describes a series of variants which essentially consist in concentrating the heating power at certain locations on a coated glazing unit, divided collector conductors together with layered heating regions structured by separation lines and/or areas being presented as means to this effect. In relation to FIGS. 11A/B and 12A/B, it is also already indicated that different potentials or voltages opposing one another may be applied to various sections of the collector conductors.
One particular configuration of glazing units with heating wires can be seen in the Patent Application DE 103 52 464 A1. In one embodiment, the region with the heating wires in the outer lateral corner of a trapezoidal heated window is divided into three strips electrically connected in series in which the heating current circulates in alternating directions. An additional section of collector conductor is also unavoidably provided. However, as with the aforementioned US Patent, this division into regions is only used to render the heating power uniform in this lateral region.
The Patent U.S. Pat. No. 5,182,431 also already discloses an arrangement of discrete heating conductors (produced by a screen printing process or in the form of wires) in several parallel regions which, in order to control the preferential heating of one region of the surface, also comprises a series circuit composed of groups of heating conductors through which currents flow in opposing directions.
The fundamental principles of electrotechnology also include the fact that the flow of an electrical current within a conductor always generates a magnetic field around this conductor. This magnetic field is normally much more intense than the background, relatively weak, Earth's magnetic field and is superimposed on the latter. It is not therefore surprising that a compass which is installed in a vehicle in the vicinity of an electrically-heatable glazing unit, notably of course in the field of view of the driver of the vehicle in the vicinity of the windshield, is perturbed and/or deviated by the electrical magnetic field produced when the glazing unit is heated to such an extent that a reliable directional indication with respect to the Earth's magnetic field is no longer possible.
Of course, this effect of superimposition only happens in practice when the finished vehicle is put into service for the final customer, because this constellation is rather rare and the interference between the magnetic field of the electric heater and the Earth's magnetic field or the compass is not necessarily envisaged in advance.
One simple solution that could have been envisaged is to provide an opening within the heating region just in the area of the compass. However, in the case of glazing units with heating wires, such a measure represents in any case a significantly increased effort when the wires are installed. Indeed, in the case of an otherwise continuous installation of the heating region, the latter would need to be laid around the opened up area and would then be more densely disposed than elsewhere in the locations concerned. This would however result in special heated glazing units whose production would not be very economical owing to the relatively low number of vehicles equipped with a compass.
The heated glazing units with electrically-conducting coating tend to form “hot spots” in the presence of uniformity defects in the conducting coating (glazing unit transparent to radiation), in other words local over-heating at the edges of these uniformity defects, which are undesirable and which, in addition to this, may cause damage to the glazing unit in the long term, notably in the case of composite glazing units with thermoplastic adhesive layer.
It is known from the Patent DE 10 2004 038 448 B3 that glazing units provided with electrically-conducting structures can be used for absorbing and attenuating radar radiation. In contrast to the documents mentioned hereinabove, the structures do not normally have any current flowing through them or are not generally connected to a voltage source.
The Patent Application DE 101 26 869 A1 discloses a heated glazing unit with wires in which the two parallel collector conductors are disposed directly adjacent to one another close to an edge of the glazing unit, the heating wires running away while being isolated by means of the collector conductors further away from the edge.